1. Field of the Invention
The present invention relates to a field emission-type electron source for emitting electron beams by means of electrical field emission and to a manufacturing method thereof.
2. Description of the Prior Art
The inventors of the present application have already proposed a field emission-type electron source having an electrically conductive substrate, a thin metal film (surface electrode) and a strong electric field drift layer interposed between the conductive substrate and the thin metal film. The strong electric field drift layer, through which electrons injected thereto from the conductive substrate can drift, is formed by rapidly and thermally oxidizing a porous polycrystalline semi conductor layer (for example, a polycrystalline silicon layer which was made porous, namely a porous polysilicon layer) by means of a rapid thermal oxidation (RTO) process.
For example, as shown in FIG. 9, a field emission-type electron source 10xe2x80x2 (hereinafter, merely referred to xe2x80x9celectron source 10xe2x80x2xe2x80x9d) is provided with an n-type silicon substrate 1 as the conductive substrate. On the main surface of the n-type silicon substrate 1, a strong electric field drift layer 6 (hereinafter, merely referred to xe2x80x9cdrift layer 6xe2x80x9d) composed of an oxidized porous polycrystalline silicon layer (porous polysilicon layer) is formed. On the drift layer 6, a surface electrode 7xe2x80x2 composed of a thin metal film is formed. In addition, on the back surface of the n-type silicon layer 1, an ohmic electrode 2 is formed.
Where the electron source 10xe2x80x2 shown in FIG. 9 is used, the surface electrode 7xe2x80x2 is disposed in a vacuum circumstance while a collector electrode 21 is disposed so as to face the surface electrode 7xe2x80x2, as shown in FIG. 10. Then a DC voltage Vps is applied between the surface electrode 7xe2x80x2 and the n-type silicon substrate 1 (ohmic electrode 2) in such a manner that the surface electrode 7xe2x80x2 has a positive electrical potential against the n-type silicon substrate 1. On the other hand, a DC voltage Vc is applied between the collector electrode 21 and the surface electrode 7xe2x80x2 in such a manner that the collector electrode 21 has a positive electrical potential against the surface electrode 7xe2x80x2. Thus the electrons injected into the drift layer 6 from the n-type silicon substrate 1 drift through the drift layer 6, and then emitted outward from the surface electrode 7xe2x80x2 (chain lines in FIG. 10 showing flows of the electrons exe2x88x92emitted from the surface electrode 7xe2x80x2). Therefore it may be preferable that the surface electrode 7xe2x80x2 is composed of a material having a smaller work function.
In the electron source 10xe2x80x2, the current flowing between the surface electrode 7xe2x80x2 and the ohmic electrode 2 is referred to xe2x80x9cdiode current Ipsxe2x80x9d. On the other hand, the current flowing between the collector electrode 21 and the surface electrode 7xe2x80x2 is referred to xe2x80x9cemitted electron current Iexe2x80x9d. The larger the ratio of the emitted electron current Ie to the diode current Ips (Ie/Ips) becomes, the higher the electron-emitting efficiency becomes. In the electron source 10xe2x80x2, even if the DC voltage Vps applied between the surface electrode 7xe2x80x2 and the ohmic electrode 2 is such a lower one as about 10 to 20V, the electrons can be emitted. According to the electron source 10xe2x80x2, the electron-emitting property less depends on the degree of the vacuum. In addition, a popping phenomenon does not occur when the electrons are emitted. In consequence, the electrons can be stably emitted with higher electror-emitting efficiency.
As shown in FIG. 11, it may be considered that the drift layer 6 includes at least polycrystalline silicon columns 51, thin silicon oxide films 52, fine crystalline silicon particles 63 of nanometer order scale and silicon oxide films 64 acting as insulating layers. The thin silicon oxide films 52 are formed on the surfaces of the polycrystalline silicon columns 51. The fine crystalline silicon particles 63 are interposed among the polycrystalline silicon columns 51. The silicon oxide films 64, each of which has the thickness smaller than the crystalline particle diameter of the fine silicon particle 63, are formed on the surfaces of the fine crystalline silicon particles 63.
That is, in the drift layer 6, it may be considered that the surface portion of each of the grains is made porous while the inner portion (core) of the grain maintains a crystalline state. Therefore the most part of the electrical field, which is applied to the drift layer 6, may be applied to the silicon oxide films 64. In consequence, the injected electrons are accelerated among the polycrystalline silicon columns 51 by the strong electric field applied to the silicon oxide films 64, and then drift in the direction of the arrow A in FIG. 11 (upward in FIG. 11) toward the surface of the drift layer 6. Thus the electron emitting efficiency may be improved. Hereupon, it may be considered that the electrons, which have reached the surface of the drift layer 6, are hot electrons so that they easily tunnel the surface electrode 7xe2x80x2, and then are emitted into the vacuum circumstance. The thickness of the surface electrode 7xe2x80x2 may be set to about 10 to 15 nm.
Meanwhile, in order to improve the electron emitting efficiency of the above-mentioned electron source 10xe2x80x2, it is necessary to restrain the electrons from scattering in the surface electrode 7xe2x80x2. Therefore the surface electrode 7xe2x80x2 is required to have characteristics as follows. That is, the surface electrode 7xe2x80x2 must restrain the electrons from scattering in the thin metal film thereof. In addition, it must have higher adhesion with the under layer (drift layer 6 in the above-mentioned case) not so as to cause its peeling during the photolithography process, the annealing process or the like. So it may be suggested such an electron source in that the surface electrode 7xe2x80x2 is composed of a first metal layer formed on the drift layer 6 and a second metal layer formed on the first metal layer, the two layers being stratified (stacked) together. Hereupon the first metal layer is composed of a metal material with higher adhesion while the second metal layer is composed of a metal material in which the electrons less scatter. In the above-mentioned electron source, however, it may be caused such a disadvantage that the electrons highly scatter in the surface electrode 7xe2x80x2 as same as the case that the surface electrode 7xe2x80x2 is composed of only one metal material in which the electrons highly scatter so that the electron emitting efficiency may be lowered, because the electrons highly scatter in the metal material with higher adhesion (probability of scattering being larger). In addition, it may be caused such a disadvantage that if the surface electrode 7xe2x80x2 is peeled off from the drift layer 6 during the manufacturing process thereof, its yield is lowered to increase its cost while its stability for the lapse of time and reliability may be lowered. The above-mentioned disadvantages may occur also in other field emission-type electron sources, for example, such as a MIM (Metal Insulator Metal) type one or a MOS (Metal Oxide Semiconductor) type one.
The present invention, which has been achieved to solve the above-mentioned problems, has an object to provide an inexpensive field emission-type electron source having good stability for the lapse of time, in which the deterioration of electron emitting efficiency due to scattering of the electrons is less, and to Provide a manufacturing method of the field emission-type electron source.
A field emission-type electron source (hereinafter, merely referred to xe2x80x9celectron sourcexe2x80x9d) according to the present invention which is performed to achieve the above-mentioned object, includes an electrically conductive substrate (hereinafter, merely referred to xe2x80x9cconductive substratexe2x80x9d), a strong electric field drift layer (hereinafter, merely referred to xe2x80x9cdrift layerxe2x80x9d) formed on a surface of the conductive substrate, and an electrically conductive thin film (hereinafter, merely referred to xe2x80x9cconductive thin filmxe2x80x9d) formed on the drift layer. In the electron source, electrons injected into the drift layer from the conductive substrate, drift in the drift layer to be emitted outward through the conductive thin film by applying a voltage between the conductive thin film and the conductive substrate in such a manner that the conductive thin film acts as a positive electrode against the conductive substrate. Hereupon, the conductive thin film has low density of states in an energy region near energy of the emitted electrons, and at least one of high adhesion for the drift layer and high sublimation enthalpy.
In the electron source, the electron emitting efficiency may be improved because the electrons, which have drifted in the drift layer, less scatter. In addition, it may be prevented that the conductive thin film is peeled off from the drift layer. In consequence, the stability for the lapse of time, of the electron source may be improved while the yield of the electron source may be raised. Therefore the cost of the electron source may be lowered.
In the above-mentioned electron source, it is preferable that the conductive thin film is composed of a metal layer including at least two metal materials, in which electrons in d-orbits of the metal materials produce a hybrid orbit so as to lower density of states of the metal layer in the energy region near energy of the emitted electrons. In this case, the conductive thin film may have lower density of states in the energy region near energy of the emitted electrons so that the electrons may be more effectively restrained from scattering.
It is more preferable that the metal layer includes a first metal material having at least one of high adhesion for the drift layer and high sublimation enthalpy, and a second metal material whose density of states in the energy region near energy of the emitted electrons is lower than that of the first material. Hereupon, the density of states of the metal layer in the energy region near energy of the emitted electrons may be also lower than that of the first material.
In the electron source, the drift layer may be composed of a porous material. It is preferable that the porous material includes, at least, polycrystalline silicon columns, fine crystalline silicon particles of nanometer order scale interposed among the polycrystalline silicon columns, and insulating films formed on surfaces of the fine crystalline silicon particles, each of the insulating films having a thickness smaller than the crystalline particle diameter of the fine silicon particle. In this case, the electron emitting property less depends on the vacuum in the circumstance while the popping phenomenon may not be caused when the electrons are emitted. In consequence, the electrons may be stably emitted with higher efficiency.
In the electron source, it is preferable that the metal layer includes a metal material having high adhesion for the drift layer and/or high sublimation enthalpy. In this case, the stability for the lapse of time, of the conductive thin film itself may be improved.
In the electron source, it is most preferable that the metal layer includes a metal material in which a first metal, which has high adhesion for the drift layer and/or high sublimation enthalpy, and a second metal, whose density of states in the energy region near energy of the emitted electrons is low, are mixed together in an atomic level to form an alloy, or chemically combined together to form a compound.
In the most preferable electron source, the electron emitting efficiency may be highly improved because the electrons, which have drifted in the drift layer, hardly scatter. In addition, it may be effectively prevented that the conductive thin film is peeled off from the drift layer. Therefore, the stability for the lapse of time and yield of the electron source may be highly improved.
In the electron source, the metal layer may include at least Au or at least Cr. If it includes Au, the electron source may have higher resistance to oxidation and higher stability for the lapse of time. On the other hand, if the conductive thin film includes Cr, it may have higher adhesion for the drift layer.
According to another aspect of the present invention, there is provided an electron source including (i) a first electrode, (ii) a surface electrode composed of an electrically conductive thin film, the surface electrode acting as a second electrode, (iii) and a drift layer disposed between the first electrode and the surface electrode, in which electrons pass through from the first electrode to the surface electrode due to an electrical field which is generated when an voltage is applied between the first electrode and the surface electrode in such a manner that the surface electrode has a higher electrical potential in comparison with the first electrode. Hereupon, the electrically conductive thin film includes a first material having at least one of high adhesion for the drift layer and high sublimation enthalpy, and a second material whose density of states in the energy region near energy of the emitted electrons is lower than that of the first material, the density of states of the thin film in the energy region near energy of the emitted electrons being lower than that of the first material.
A method of manufacturing the above-mentioned most preferable electron source includes the steps of attaching at least the first and second metals to the drift layer, and performing a stabilizing treatment for alloying or chemically combining the first and second metals together to form the metal layer.
The conductive thin film of the electron source manufactured by the method may have such higher adhesion that it is not peeled off during the manufacturing process, for example during the photolithography process. In addition, the conductive thin film may have higher electron emitting efficiency. Therefore the electron source may have excellent stability for the lapse of time while the cost of the electron source may be lowered. Moreover, it is possible to use a material composed of a simple substance during the film forming process. Therefore it is not necessary to consider the composition of the materials during the film forming process. In consequence, the cost of the electron source may be further lowered while the manufacturing process may be simplified.
In the above-mentioned method, the stabilizing treatment may be performed by applying UV rays to a surface of the metal disposed at an outermost position. In this case, the first and second metals may be alloyed or chemically combined without causing a breakdown of the device.
The stabilizing treatment may be performed while applying ozone to the surface of the metal disposed at the outermost position. In this case, also, the first and second metals may be alloyed or chemically combined without causing a breakdown of the device. In addition, it may be prevented that the electron emitting efficiency is lowered due to contamination by organic substances. Therefore the electron source may have much higher electron emitting efficiency.
Further, the stabilizing treatment may be performed by applying UV rays to a surface of the metal disposed at the outermost position while heating the first and second metals. In this case, the time, which is required for alloying or chemically combining the first and second metals, may be shortened. Therefore its throughput may be improved.
Moreover, the stabilizing treatment is performed by applying UV rays and ozone to the surface of the metal disposed at the outermost position while heating the first and second metals. In this case, the metal layer may be prevented from being contaminated by organic substances. In consequence, it may be prevented that the electron emitting efficiency is lowered due to the contamination by the organic substances. Therefore the electron emitting efficiency of the electron source may be more highly improved.
In the above-mentioned method, the first and second metals may be attached to the drift layer by stratifying the first and second metals onto the drift layer. For example, the first and second metals may be stratified using an alternate sputtering process or a vapor deposition process. In this case, as the film forming process, it is possible to utilize a general process which has been used in the process for manufacturing semiconductor.
Hereupon, the stratified first metal may be formed on the drift layer while the stratified second metal is formed at a position nearest to the surface electrode, during the stratifying step.
Meanwhile, in the above-mentioned method, the first and second metals may be attached to the drift layer in such a state that the first and second metals are mixed together. For example, the first and second metals may be attached to the drift layer by simultaneously sputtering or depositing the first and second metals onto the drift layer. In this case, the conductive thin film of the electron source may have such higher adhesion that it is not peeled off during the manufacturing process, for example during the photolithography process. In addition, the conductive thin film may have higher electron emitting efficiency. Therefore the electron source may have an excellent stability with the lapse of time while the cost of the electron source may be lowered. If the sputtering or depositing process is used, the time required for the film forming process may be shortened. In consequence, its throughput may be improved so that its manufacturing cost may be lowered.
Another method of manufacturing the above-mentioned most preferable electron source includes the step of attaching vapor or fine particles made from a source or target in which the first and second metals have been alloyed or chemically combined, to the drift layer to thereby form the metal layer. For example, the fine particles or vapor of the target may be attached to the drift layer by sputtering or depositing the target onto the drift layer. In this case, the conductive thin film of the electron source may have such higher adhesion that it is not peeled off during the manufacturing process, for example during the photolithography process. In addition, the conductive thin film may have a higher electron emitting efficiency. Therefore the electron source may have excellent stability for the lapse of time while the cost of the electron source may be lowered. Moreover, if the sputtering or depositing process is used, the time required for the film forming process may be shortened. In consequence, its throughput may be improved so that its manufacturing cost may be lowered.
A further method of manufacturing the above-mentioned most preferable electron source includes the step of attaching at least the first and second metals, which are formed in such small sizes that the first and second metals car be naturally alloyed or chemically combined together, to the drift layer to form the metal layer. For example, the first and second metals may be attached to the drift layer in such a state that thin layers of the first metal and thin layers of the second metal are alternately stratified, or that fine particles of the first metal and fine particles of the second metal are mixed together. In this case, the conductive thin film of the electron source may have such higher adhesion that it is not peeled off during the manufacturing process, for example during the photolithography process. In addition, the conductive thin film may have higher electron emitting efficiency. Therefore the electron source may have excellent stability with the lapse of time while the cost of the electron source may be lowered. Moreover, the time required for the film forming process may be shortened. In consequence, its throughput may be improved so that its manufacturing cost may be lowered.